As is well known, a variety of laboratory test procedures involve the dropwise addition of reagents to institute, maintain, or interrupt the reactions. A typical test, used in the field of immunohematology and blood banking, is the Coombs antihuman globulin test in which one drop (in the indirect Coombs test; two drops in the direct test) of antihuman serum, also known as Coombs serum, prepared from the blood of animals which have been immunized against purified human globulin, is added to a sample of red blood cells to be tested for the purpose of promoting visible agglutination of those erythrocytes which have become coated or sensitized by prior immunologic reaction. If agglutination occurs, it demonstrates the occurrence of a previous reaction between the blood cell antigens and antibody. The test may be used in preparing blood for use in transfusion, with donor's cells and recipient's serum being mixed to ascertain if a combination of antigen and antibody exists, a positive reaction after the addition of antihuman serum indicating incompatibility.
It is believed apparent that a false negative reaction could have serious consequences since it would constitute an incorrect indication of blood compatibility. While automatic analysis equipment is now used with increasing frequency in laboratories because of the greater speed and reliability of automated procedures in contrast to manual procedures, there is a continuing danger that malfunctioning of the equipment might give misleading results. For example, in the operation of equipment designed to carry out the Coombs test, false negative reactions would be expected to occur if for some reason the drop of Coombs serum was not added to each test tube in the final stages of the procedure, or if for some reason an insufficient amount of serum were so added. The problem is further complicated by the fact that upon exposure to air, Coombs serum becomes increasingly viscous and may clog the passage of the drop-dispensing mechanism. While the problem is particularly well illustrated by the Coombs test, and by equipment designed to perform that test automatically, the same problem exists in varying degrees in the automation of other laboratory tests requiring dropwise addition of reagents.
The specific problems described above have not been fully solved in the past despite considerable effort in that regard. Photoelectric devices have previously been known and used for detecting the presence or absence of fluids but such devices, along with other safety measures, have not resulted in automated drop-dispensing laboratory equipment which is sufficiently reliable, non-clogging and substantially maintenance-free. U.S. Pat. Nos. 3,225,191, 3,454,759, 2,446,885, 3,038,300, 3,418,053, 2,718,597, and 3,548,193 are illustrative of the prior art.